In the Hunger Games, when your ages 12-18 your name is put down to enter (against your will but you can enter a 2nd time and they will give your family some food to get by) and this happens each year.
And, then the annual Hunger Games begin, an escort from the Capitol will pick a boy and a girl from the entries, where, as mentioned above, will be forced to partake in a Battle Royale for the entertainment of others. This is known as reaping.